This invention is generally directed to imaging members, and more specifically the present invention relates to the use of certain electron transporting compounds which function as protective overcoatings for photoresponsive imaging members. In one embodiment, the present invention relates to an imaging member comprised of an inorganic photoconductive composition, and coated thereover electron transport polyurethane polymers. Also, in another embodiment of the present invention, there are provided imaging members comprised of a photogenerating layer, a hole transport layer, and overcoated thereover as an electron transporting protective overcoating a polyurethane polymer. The aforementioned imaging members are useful in electrostatographic imaging processes, and in particular, can be selected for the generation of latent images in electrostatic imaging systems.
The specific electron transport overcoating polyurethane polymers of the present invention, when selected for the imaging members, disclosed perform a variety of functions inclusive of providing protection for the aforementioned members from abrasive physical and chemical contamination. Accordingly, thus for example, the overcoating polyurethane polymers of the present invention permit the resulting imaging member to be resistant to ozone and other chemical substances produced by corona charging devices. Also, the polyurethane electron transport overcoating polymers of the present invention substantially eliminate undesirable scratching of the imaging members involved, and further these coatings can function as release materials permitting the excellent removal and transfer of toner images. Futhermore, the polyurethane electron transport coatings of the present invention can be easily formulated as discrete layers and remain essentially nonreactive to the ink/solvent formulation utilized for certain liquid ink xerographic development processes. Moreover, the protective electron transporting overcoatings of the present invention are non-toxic and are, therefore, inert to users of the device.
It is known that the application of protective coatings to certain photoconductive materials, particularly inorganic photoconductive materials, is designed primarily for the purpose of extending the useful life of the resulting devices. Generally, in order for these coatings to provide the desired protection they should possess certain mechanical properties, and must be applied in a substantially uniform thickness. Additionally, the coating material should be selected so as to not adversely affect the photoelectric properties of the photoreceptor, for example, the coating should not appreciably inject charges in the dark. The protective coatings should also not conduct laterally on the overcoat surface. Further, in some applications the coating must be transparent, and possess a dark resistivity at least equal to the dark resistivity of the photoconductive material. For example, photoconductive materials such as selenium have a resistivity in the dark of 10.sup.10- 10.sup.12 ohm-cm, thus the dark resistivity of the protective coating should be in this range when it is used as a protectant for selenium. In addition, the coatings should not be sensitive to changes in humidity and temperature otherwise the photoelectric properties of the protected photoreceptors will change with humidity.
With regard to vitreous selenium, the most widely used photoconductive material, it suffers from two serious defects, namely, its spectral response is somewhat toward the blue or near ultraviolet, and the preparation of uniform films of vitreous selenium has required highly complex processes wherein critical parameters are involved. Accordingly, from a commercial economic aspect, it is important that xerographic selenium devices be utilized for numerous imaging cycles. The overcoatings of the present invention enable this objective to be achieved.
Deterioration by mechanical abrasion attendant to the developing and the cleaning processes, wherein in one cleaning process a rapidly rotating brush contacts the photoconductive surface for the purpose of removing therefrom any residual developer particles adhering thereto subsequent to the transfer step, has been observed in selenium. In addition to mechanical abrasion, the selenium photoreceptor may be subjected to intense heat, which over a period of time adversely effects its photoconductivity. Accordingly, and for other reasons inclusive of preventing crystallization of selenium upon exposure to solvent vapors, various protective coatings, or overcoatings have been applied to selenium devices. Thus, there is described in U.S. Pat. No. 3,397,982 an electrostatographic device comprising a photoconductive layer including an inorganic glass material, the photoconductive layer containing an overcoating comprised of various oxides, such as germanium oxides, the oxides of vanadium, and silicon dioxide.
Additionally, in U.S. Pat. No. 2,886,434 there are disclosed processes for the protection of selenium photoconductive substances with a thin transparent film of a material having electrical characteristics equal to selenium. Examples of materials disclosed as a protective layer for selenium include zinc sulfide, silica, various silicates, alkaline earth fluorides, and the like. Furthermore, there is disclosed in U.S. Pat. No. 2,879,360 a photoconductor comprising a support substrate, a layer of photoconductive material, and as a protectant a thin film of silicon dioxide superimposed upon the photoconductive layer.
Also, there are illustrated in the prior art photoresponsive devices comprised of a conductive substrate, overcoated with a hole injecting layer, which in turn is overcoated with a hole transport layer, followed by a carrier generating layer, and an insulating organic resin as a top coating. These devices have been found to be very useful in various imaging systems, and have the advantage that high quality images are obtained with the overcoating acting primarily as a protectant. Another similar overcoated photoresponsive device is comprised of a conductive substrate layer, a generating layer, and a transport layer. In such devices, the generating layer can be overcoated on the transport layer, or the transport layer may be overcoated on the generating layer. Examples of such devices are described in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference.
Additionally, there is disclosed in U.S. Pat. No. 4,423,131, entitled Photoresponsive Devices Containing Polyvinylsilicate Coatings, improved photoresponsive imaging members with a protective overcoating top layer of a crosslinked polyvinylsilicate resulting from the reaction of polysilicic acid with a polyvinyl alcohol with a number average molecular weight of from about 10,000 to about 100,000.
Several of the above-described overcoated organic photoresponsive devices are not effectively protected after extended usage, and in some instances the imaging properties thereof are adversely affected subsequent to a few imaging cycles. More specifically, with these devices the properties of the top overcoating material, or the properties of the other layers are adversely effected by ozone, and other contaminants present in the environment by the developing compositions which contact the photoresponsive device for the purpose of rendering the image visible, and mechanical abrasion during cycling. Accordingly, images of low quality, or no images whatsoever are produced depending upon the extensiveness of the damage caused to the layers of the photoconductive device selected. Furthermore, in some instances, the toner materials employed do not sufficiently release from the photoresponsive surface, leaving unwanted toner particles thereon causing them to be subsequently embedded into, or transferred from the imaging surface in later imaging steps thereby resulting in undesirable images of low quality, and/or high background. Also, in some instances, the dried toner particles adhere to the imaging member and print out as background areas. This can be particularly troublesome when known silicone resins or elastomeric polymers are employed as overcoating materials for their melted toner release characteristics, since any low molecular weight components contained in these polymers can migrate to the surface of the silicone polymer layer, and act as an adhesive for dry toner particles brought in contact therewith during image development. There thus results undesirable high background areas in the final image since toner particles together with the developed images are effectively transferred to the receiving sheet.
Furthermore, disclosed in a copending application U.S. Ser. No. 672,749, entitled Photoresponsive Imaging Members Containing Electron Transport Overcoatings, are imaging members comprised of a supporting substrate, a hole transport layer comprised of an arylamine hole transporting compound dispersed in an inactive resinous binder, a photogenerating layer comprised of a photogenerating pigment optionally dispersed in a resinous binder, and as a protective topcoating an electron transporting layer compound of the following formula dispersed in a resinous binder ##STR2## where X is cyano or alkoxycarbonyl groups, A and B are electron withdrawing groups, m is a number of from 0 to 2, n is the number 0 to 1, and W is an electron withdrawing group selected from the groups consisting of acyl (COR), alkoxycarbonyl (COOR), alkylaminocarbonyl (CONHR), and derivatives thereof.
Furthermore, other prior art includes U.S. Pat. Nos. 4,474,865, which describes improved photoresponsive imaging members with electron transporting components containing specific dicyano fluoro ester moieties; 3,928,034, which illustrates the incorporation of electron transporting moieties chemically attached to polymers, reference columns 7 and 8; and 4,007,043; 4,063,947; 4,075,012; and 3,896,184.
While the above-described imaging members disclosed are suitable for their intended purposes, there continues to be a need for improved protective overcoatings for incorporation into layered imaging members. More specifically, there continues to be a need for protective overcoatings which simultaneously function as electron transporting media enabling the resulting photoresponsive imaging members to be useful in xerographic imaging processes, particularly color processes, in that the members can be positively charged. Additionally, there continues to be a need for improved layered devices wherein the protective overcoating is comprised of a polymer based on the electron transporting 9-fluoroenylidenemethane structure. These coatings possess excellent toner release properties, and are impermeable to chemical materials produced by corona charging devices. Also, there continues to be a need for insulating protective overcoatings which simultaneously function as an electron transporting media, and wherein these overcoatings are not conductive to charges applied by a corona charging device. Furthermore, there remains a need for electron transport overcoatings which are mechanically strong and durable while simultaneously being insensitive to the effect of humidity. Also, there is a need for heat resistant overcoatings for layered photoresponsive imaging members which are capable of protecting these members from direct exposure to heat without adversely affecting their imaging performance. There also remains a need for protective overcoatings which prevent the escape of toxic materials, especially inorganic materials such as arsenic and tellurium from photoreceptor imaging members. Moreover, there is a need for protective overcoatings that will prevent photoconductors such as selenium from crystallization upon exposure to solvent vapors.
Further, there continues to be a need for new protective overcoatings for inorganic photoconductive members inclusive of members comprised of selenium and selenium alloys. Also, there is a need for reliable single component protective overcoatings for layered imaging members, which coatings have several desirable characteristics including toughness, high durability, and enable the transporting of electrons.